Added RPN version of 24 game

An alternate implementation of the 24 game problem which uses RPN (reverse polish notation expressions). 

FYI [RosettaCode](http://rosettacode.org/wiki/Reports:Tasks_not_implemented_in_Rust) still shows that 24 game and 24 game solve are not yet completed.
two nits:

1. it would be better to return a `Result<(), String>`  here. Instead of printing the results directly from the function you could handle them from the calling function.

2. input parameters like `expr` are usually a `&str`, not a `String` (they avoid having to allocate a `String` before calling the function, unless it's strictly needed)